Dimdim is a free, open-source conferencing and screen-sharing app. It's a simple tool for sharing presentations, collaborating on projects, or having meetings between people in multiple locations.
You can use most of Dimdim's features without having to pay, or install anything on your computer. However, with a small plug-in you can share your entire computer screen with other meeting participants.
Because it's open-sourced, Dimdim's been integrated into several third-party services like Zimbra and SugarCRM, and has benefited from community bug-bashing.